This invention relates to an image forming system, and particularly to a temperature control of the image forming system.
In an image forming system, a latent image which corresponds to an image of a manuscript is formed in a photosensitive material formed on a photosensitive drum, toners are absorbed to the photosensitive material, then the toners are transferred to a recording medium such as a paper, and further the toners are thermally fixed by heating means to obtain a transcribed image.
In order to perform such thermal fixing, in the image forming system, there is usually provided a fixing unit which includes a fixing roller having a smooth surface and heating means, a pressing roller to press a paper to be transcribed against the fixing roller. Papers to which toners are transferred are passed between the fixing roller and the pressing roller.
Temperature control for the fixing roller is extremely important to obtain quality transcribed images.
For this purpose, the following prior arts are known.
The first one is disclosed in Japanese Patent Laid-open Publication No. 2000-315034, which provides a first temperature detecting means for detecting surface temperature of the fixing roller; a first controlling means for ON/OFF controlling of a first heating means for heating the fixing roller to maintain a setting temperature in response to the detection result of the first temperature detecting means; a non-contact heat supply means having a second heating means for heating and melting the image toners on the paper; a second temperature detecting means for detecting temperature of said second heating means, and a second controlling means for ON/OFF controlling said second heating means at a setting temperature in response to the detection result of the first temperature detecting means so that the temperature of said non-contact heat supply means has a predetermined relation with the surface temperature of said fixing roller.
However, if the first temperature detecting means used here is of the type that touches the fixing roller, there might be a trace of the first temperature detecting means on the fixed image. Furthermore, since toners easily attach to this first temperature detecting means and the second temperature detecting means which detects the temperature of the non-contact heat supply means, it is difficult to precisely control the fixing temperature. Still further, if the second temperature detecting means is located above the heating rollers, the temperature detected will rise and the temperature will be detected incorrectly due to heat convection.
The second prior art is disclosed in Japanese Patent Laid-open publication No. 10-39676 (1998), in which the fixing unit is constructed of a cylinder member having a core and an exciting coil wound on the core, and a heat detecting means which detects temperature is provided at an opposite position with the cylinder member being disposed between the exciting coil and the detecting means.
In this prior art, there are still problems that toners may attach to the heat detecting means, and in the case where its position is above the heating roller, temperature may be incorrectly detected due to heat convection.